netdev - Re: [PATCH RFC] spinlock: split out debugging check from spin_lock

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Date:	Thu, 11 Apr 2013 12:56:30 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <erdnetdev@...il.com>
Cc:	Bart Van Assche <bvanassche@....org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] spinlock: split out debugging check from
 spin_lock_mutex

On Thu, Apr 11, 2013 at 08:57:01AM -0700, Eric Dumazet wrote:
> On Thu, 2013-04-11 at 11:18 -0400, Neil Horman wrote:
> > Bart, this patch should fix your problem.  Could you please test it and confirm?
> > 
> > Bart Van Assche recently reported a warning to me:
> > 
> > <IRQ>  [<ffffffff8103d79f>] warn_slowpath_common+0x7f/0xc0
> >  [<ffffffff8103d7fa>] warn_slowpath_null+0x1a/0x20
> >  [<ffffffff814761dd>] mutex_trylock+0x16d/0x180
> >  [<ffffffff813968c9>] netpoll_poll_dev+0x49/0xc30
> >  [<ffffffff8136a2d2>] ? __alloc_skb+0x82/0x2a0
> >  [<ffffffff81397715>] netpoll_send_skb_on_dev+0x265/0x410
> >  [<ffffffff81397c5a>] netpoll_send_udp+0x28a/0x3a0
> >  [<ffffffffa0541843>] ? write_msg+0x53/0x110 [netconsole]
> >  [<ffffffffa05418bf>] write_msg+0xcf/0x110 [netconsole]
> >  [<ffffffff8103eba1>] call_console_drivers.constprop.17+0xa1/0x1c0
> >  [<ffffffff8103fb76>] console_unlock+0x2d6/0x450
> >  [<ffffffff8104011e>] vprintk_emit+0x1ee/0x510
> >  [<ffffffff8146f9f6>] printk+0x4d/0x4f
> >  [<ffffffffa0004f1d>] scsi_print_command+0x7d/0xe0 [scsi_mod]
> > 
> > This resulted from my commit ca99ca14c which introduced a mutex_trylock
> > operation in a path that could execute in interrupt context.  When mutex
> > debugging is enabled, the above warns the user when we are in fact exectuting in
> > interrupt context.
> > 
> > I think this is a false positive however.  The check is intended to catch users
> > who might be issuing sleeping calls in irq context, but the use of mutex_trylock
> > here is guaranteed not to sleep.
> > 
> > We could fix this by replacing the DEBUG_LOCK_WARN_ON check in spin_lock_mutex
> > with a __might_sleep call in the appropriate parent mutex operations, but for
> > the sake of effiency (which It seems is why the check was put in the spin lock
> > code only when debug is enabled), lets split the spin_lock_mutex call into two
> > components, where the outer component does the debug checking.  Then
> > mutex_trylock can just call the inner part as its callable from irq context
> > safely.
> > 
> > Signed-off-by: Neil Horman <nhorman@...driver.com>
> > Reported-by: Bart Van Assche <bvanassche@....org>
> > CC: Bart Van Assche <bvanassche@....org>
> > CC: David Miller <davem@...emloft.net>
> > CC: netdev@...r.kernel.org
> 
> Please post your patch to lkml ?
> 
I will, I just wanted Bart to test it first, and CCed everyone that he sent his
origional note too.

Thanks
Neil

> Thanks !
> 
> 
> 
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